Printed circuit boards (PCB) are commonly used in the manufacture of digital devices, automated testing equipment (ATE) and in a range of general electronic equipment. Each PCB has a variety of electronic components mounted thereon. Examples of such components include semiconductor chips, integrated circuits (IC), resistors, transistors and capacitors. The electrical connection between each mounted component and the PCB (which may also be referred to as a substrate), is commonly taken to be part of the semi-conductor packaging process and is of paramount importance. An improper or faulty electrical connection between the electrical component and the PCB is likely to result in the functional failure of the device or equipment in which the faulty connection occurs.
Amongst the above-mentioned components that are commonly mounted onto a PCB, the IC may be considered to be one of the most important. In mounting the IC onto the PCB, manufacturers have typically relied on three techniques, namely, wire bonding, tape automated bonding (TAB) and flip-chip mounting technology. Amongst the three techniques of mounting an IC, wire bonding is the most commonly used method. In wire bonding, the PCB (or substrate) has a plurality of bonding pads situated thereon in a predetermined pattern. The IC, having electrical leads, is typically mounted in the center of the predetermined pattern of bonding pads and its electrical leads are then connected to the bonding pads. The electrical connections between the IC and the PCB are established by using either copper, gold or aluminum wires having diameters typically between the range of about 10-about 200 microns, depending upon the requirements of the circuit and bond pad size. The wires are usually attached with one end to the electrical lead of the mounted IC, then drawn towards the respective bonding pad, and are finally attached to the appropriate bonding pad thereby establishing an electrical connection between the substrate and the IC mounted thereon.
The establishment of the electrical connection between the mounted IC and its respective substrate, as described above, is carried out by a bonding tool. The bonding tool supplies the fine wire and employs a method known as ball bonding to electrically connect the electrical leads of the IC to the bond pads of the substrate. The bonding tool comprises a capillary through which the fine wire is threaded through. The capillaries are typically made from a ceramic material such as aluminum oxide, tungsten carbide and aluminum toughened zircon, for example. An example of such a conventional wire bonding tool is described in U.S. Pat. No. 6,910,612. This U.S. patent describes a bonding tool having a cylindrical axial passage coupled to a working tip having an inner annular chamfer. The annular chamfer has a predetermined angle and face length, which essentially go towards shaping the ball bond during the bonding process.
The process of wire bonding involves having the wire threaded through the capillary and leaving a free end of the wire at the working tip of the capillary. The free end of the wire at the working tip of the capillary is the end that forms the ball for ball bonding, which is a form of wire bonding. As an exemplary illustration, when a gold fine wire is used, the process is known as gold ball bonding. During gold bond bonding, a gold ball is first formed by melting the end of the wire at the capillary tip through electronic flame-off (EFO). This gold ball, which is generically known as a free-air ball, typically has a diameter ranging from 1.5 to 2.5 times that of the wire diameter. The free-air ball size consistency is controlled by the EFO. The free-air ball is then brought into contact with the bond pad of the substrate or the electrical lead of the mounted IC. When the free-air ball contacts the bond pad, for example, adequate amounts of pressure, heat, and ultrasonic forces are then applied to the ball bond for a specific amount of time, thus forming the initial metallurgical weld between the ball and the bond pad, as well as deforming the ball bond itself into its final shape. The wire is then run to the electrical lead corresponding to the bond pad to create a gradual arc or “loop” between the bond pad and the electrical lead. Pressure and ultrasonic forces are applied to the wire to form the second bond (known as a wedge bond, or stitch bond) with the electrical lead in order to complete one bonding cycle.
With regard to the application of ultrasonic energy, the effect of such energy causes the bonding tool, in particular, the tip of the capillary, to oscillate. Accordingly, when ultrasonic energy is applied to the bonding tool after the ball bond is formed, the bonding tool, in effect, scrubs the ball bond against the bond pad. This scrubbing action cleans the bond pad, which is typically aluminum, of debris and oxides, such as aluminum oxide, for example. This scrubbing action exposes a fresh surface of the bond pad in the process. The metallurgical bond or weld between the ball bond and the bond pad is further enhanced with the continued application of ultrasonic energy, resulting in plastic deformation of the ball bond and bond pad against each other. Aside from the physical contact and deformation of the metals unto each other, inter-diffusion of the ball bond and bond pad metal atoms also occurs, which further enhances the metallurgical bond. In general, bond reliability increases with the level of inter-diffusion that takes place. The most common reason for insufficient inter-diffusion is the presence of foreign materials or contaminants on the surface of the bond pad, such as oxides, unetched glass, silicon saw dust, and process residues, for example. The importance of the application of sufficient ultrasonic energy to achieve a reliable bond is underscored by the need to also ensure that the bond pad is free of contaminants.
Presently, there is a growing trend in the semiconductor industry to use materials having sensitive metallization such as materials with low dielectric constants (low K value), for example. Materials having a sensitive metallization also include ultra-thin bond pads. Generally, materials with such sensitive metallization have poor mechanical properties, low thermal conductivity and are more susceptible to metal peel-off, cratering and oxide cracks during wire bonding.
Another growing trend in the industry is the carrying out of ‘bonding over active circuitry’. This requires a stable wire bonding process especially since ‘bonding over active circuitry’ essentially involves forming a metal bond pad with a metal layer thereon over circuitry in the semiconductor chip.
Bonding tools, where the amount of combined mechanical stress from the applied ultrasonic energy, impedance and the impact force of said bonding tool on the bond pad may result in serious damage occurring to the bond pads with sensitive metallization. Furthermore, such conventional bonding tools may not be able to form bonds reliable enough for carrying out ‘bonding over active circuitry’.
Typically, when a conventional bonding tool, such as that described in the aforesaid U.S. Pat. No. 6,910,612, is used in conjunction with a substrate of a low K value, the ultrasonic energy delivered at the working tip of conventional bonding tools is usually insufficient to bond low K bond pads through metallization. The working tip of the conventional capillary requires higher power ultrasonic settings to bond sufficiently with a bond pad of low K value. However, if a higher ultrasonic energy is supplied, it tends to further aggravate the above-mentioned problems of metal peel-off, cratering and oxide cracks.
In response to the above-mentioned problem concerning materials having a low-K value, U.S Pat. No. 6,321,969 discloses a wire bonding tool that is capable of a more efficient transfer of ultrasonic energy from the ultrasonic source to the working tip of the tool. As mentioned above, apart from scrubbing oxide of a bond pad, the oscillating action of the working tip, when ultrasonic energy is applied to it, aids in the inter-diffusion of the bond and bond pad metal atoms thereby further enhancing the metallurgical bond. Accordingly, U.S. Pat. No. 6,321,969 discloses that if a more efficient transfer of ultrasonic energy to the working tip takes place, a lower amount of said ultrasonic energy may then be applied to oscillate the working tip, thereby avoiding the aforesaid difficulties associated with bonding materials having a low-K value.
However, there is still a need for a wire bonding tool capillary that is capable of forming reliable wire bonds with substrates having sensitive metallization, and for carrying out ‘bonding over active circuitry’. There is also a concurrent need for the bonding tool to be easily integrated into existing wire bonding facilities and to be cost-effective as well. In this respect, the wire bonding tool as described in detail hereafter overcomes the aforesaid difficulties.